Photovoltaic cells have developed according to two distinct methods. A first form produces cells employing a matrix of crystalline silicon appropriately doped to produce a planar p-n junction. An intrinsic electric field established at the p-n junction produces a voltage by directing solar photon produced holes and free electrons in opposite directions. Good conversion efficiencies and long-term reliability have been demonstrated for crystalline silicon cells. However, widespread energy collection using crystalline silicon cells is thwarted by the high cost of crystal silicon (especially single crystal silicon) material and interconnection processing.
A second approach to produce photovoltaic cells is by depositing thin photovoltaic semiconductor films on a supporting substrate. Many various techniques have been proposed for deposition of semiconductor thin films. The deposition methods include vacuum vapor deposition, vacuum sputtering, electroplating, chemical vapor deposition and printing of nanoparticle inks. These structures have become know in the art as “thin film” devices. Material requirements are minimized and technologies can be proposed for mass production. Typical semiconductors used for thin film photovoltaic devices include cuprous sulfide, cadmium telluride (CdTe), copper-indium-gallium-diselenide (CIGS), amorphous silicon, printed silicon, and dye sensitized polymeric materials. The thin film structures can be designed according to doped homojunction technology or can employ heterojunction approaches such as those using CdTe or chalcopyrite materials.
Despite significant improvements in individual cell conversion efficiencies for both single crystal and thin film approaches, photovoltaic energy collection has been generally restricted to applications having relatively low power requirements. One factor impeding development of bulk power systems is the problem of economically collecting the energy from an extensive collection surface. Photovoltaic cells can be described as high current, low voltage devices. Typically individual cell voltage is less than about two volts, and often less than 0.6 volt. The current component is a substantial characteristic of the power generated. Efficient energy collection from an expansive surface must minimize resistive losses associated with the high current characteristic. A way to minimize resistive losses is to reduce the size of individual cells and connect them in series. Thus, voltage is stepped through each cell while current and associated resistive losses are minimized.
Regardless of whether the cells are crystalline silicon or thin film, nuking effective, durable series connections among multiple small cells can be laborious, difficult and expensive. In order to approach economical mass production of series connected arrays of individual cells, a number of factors must be considered in addition to the type of photovoltaic materials chosen. These include the substrate employed and the process envisioned. A first problem which has confronted production of expansive surface photovoltaic modules is that of collecting the photogenerated current from the top, light incident surface. Transparent conductive oxide (TCO) layers are normally employed to form a top surface. However, these TCO layers are relatively resistive compared to pure metals. Thus, efforts must be made to minimize resistive losses in transport of current through the TCO layer. One approach is simply to reduce the surface area of individual cells to a manageable amount. However, as cell widths decrease, the width of the area between individual cells (interconnect area) should also decrease so that the relative portion of inactive surface of the interconnect area does not become excessive. Typical cell widths of one centimeter or less are often taught in the art. These small cell widths demand very fine interconnect area widths, which dictate delicate and sensitive techniques to be used to electrically connect the top TCO surface of one cell to the bottom electrode of an adjacent series connected cell. Furthermore, achieving good stable ohmic contact to the TCO cell surface has proven difficult, especially when one employs those sensitive techniques available when using the TCO only as the top collector electrode.
One approach to expand the surface area of individual cells while avoiding excessive resistive losses in current collection is to form a current collector grid over the surface. This approach positions highly conductive material in contact with the surface of the TCO in a spaced arrangement such that the travel distance of current through the TCO is reduced. In the case of the classic single crystal silicon or polycrystal silicon cells, a common approach is to form a collector grid pattern of traces using a silver containing paste and then fuse the paste to sinter the silver particles into continuous conductive silver paths. These highly conductive traces normally lead to a collection buss such as a copper foil strip. One notes that this approach involves use of expensive silver and requires the photovoltaic semiconductors to tolerate the high fusion temperatures. The sintering temperatures involved are normally unsuitable for thin film photovoltaic structures. Another approach is to attach an array of fine copper wires to the surface of the TCO. The wires may also lead to a collection buss, or alternatively extend to an electrode of an adjacent cell. This wire approach requires positioning and fixing of multiple fine fragile wires which makes mass production difficult and expensive. Another approach commonly used for thin film photovoltaic cells is to print a collector grid array on the surface of the TCO using a conductive ink, usually one containing a heavy loading of fine particulate silver. The ink is simply dried or cured at mild temperatures to remove a solvent carrier. Compared to the high sintering temperatures associated with the silver pastes employed with crystal silicon cells, the milder curing temperatures for silver inks typically do not adversely affect thin film photovoltaic structures. However, the silver ink approaches require the use of relatively expensive inks because of the required high loading of finely divided silver. Furthermore, batch printing on the individual cells is laborious and expensive.
In addition to current collection from the top surface of cells, efficient photovoltaic power collection includes integration of multiple cells into arrays or modules to create a desired surface area. The multiple cells are typically electrically integrated in series arrangement such that the power is accumulated in voltage increments. Regarding crystalline silicon cells, the individual cells are normally initially discrete and comprise rigid wafers approximately 200 micrometers thick and approximately 230 square centimeters in area. A conventional way to harvest power from multiple such cells is to use a conventional “string and tab” arrangement. This technique involves first depositing fine conductive current collecting grid fingers over the light incident surface. As previously discussed, these fingers often are in the form of a fired silver paste or fine metal wires. Multiple grid fingers lead to a robust buss of substantial current carrying capacity. This buss material then extends and is electrically joined to the bottom electrode of an adjacent cell. Such methods for electrically integrating multiple discrete cells can be termed “discrete integration”.
A typical prior art “string and tab” arrangement for achieving series connections among crystalline silicon cells is embodied in FIGS. 1A through 1C. It is in seen in FIG. 1A that conductive grid fingers 82 are attached to the light incident (top) surface 83 of cells 84. These fingers 82 extend to buss material 85 positioned at opposite peripheral edges of cells 84. The buss material extends to the bottom electrode 86 of an adjacent cell, as is shown in the bottom view of FIG. 1B and side view of FIG. 1C. It is to be noted that the busses 85 in FIGS. 1A through 1C are depicted with section lines. This is done for contrast only and the views are not actually sectional views. While FIGS. 1A through 1C show the interconnection of two cells, in reality this connection is normally made among strings of many more cells (8 for example). This process is thus laborious, costly and subject to manufacturing error. Further, the strings of cells are physically turned over in order to access both top and bottom surfaces of the individual cells to accomplish the electrical connections. Such a process may lead to breaking of electrical connections and complicates efforts to achieve a continuous high volume production process for the integrated cells.
Thin film photovoltaic semiconductors can be deposited over expansive areas and often in a continuous roll-to-roll fashion. Thin film technologies may thus offer additional opportunities for mass production of interconnected arrays compared to inherently small, discrete single crystal silicon cells. For example, thin film photovoltaic cells may be subdivided and interconnected into arrays of multiple cells using a process generally referred to as “monolithic integration”. Monolithic integration envisions initially depositing photovoltaic cell structure over an expanded surface of supporting substrate. The expansive photovoltaic structure is subsequently subdivided into smaller, isolated, individual cells which are then serially interconnected while maintaining the cells on the initial common substrate.
A number of U.S. patents have issued proposing designs and processes to achieve such monolithic series integration among thin film photovoltaic cells. Examples of these proposed processes are presented in U.S. Pat. Nos. 4,443,651, 4,724,011, and 4,769,086 to Swartz, Turner et al. and Tanner et al. respectively which taught monolithic integration techniques for photovoltaic cells supported by glass substrates. The process comprises deposition of photovoltaic materials on glass substrates followed by scribing to form smaller area individual cells. Multiple steps then follow to electrically connect the individual cells in series array. While expanding the opportunities for mass production of interconnected cell arrays compared with single crystal silicon approaches, glass substrata must inherently be processed on an individual batch basis. Further, when multiple individual cells are formed monolithically on a common monolithic glass substrate, there is no way to check the quality of individual cells and remove deficient cell regions. Thus variations in cell quality over an expansive surface may jeopardize the entire module.
More recently, developers have explored depositing wide area films using continuous roll-to-roll processing. This technology generally involves depositing thin films of photovoltaic material onto a continuously moving sheet like web of insulating plastic or metal foil. However, a challenge still remains regarding monolithically subdividing the expansive films into individual cells followed by inteconnecting into a series connected array. For example, U.S. Pat. No. 4,965,655 to Grimmer et. al. and U.S. Pat. No. 4,697,041 to Okinawa teach processes employing insulating polymeric substrates requiring expensive laser scribing and interconnections achieved with laser heat staking. In addition, these two references teach a substrate of thin vacuum deposited metal on substrate films of relatively expensive polymers. The electrical resistance of thin vacuum metallized layers may significantly limit the active area of the individual interconnected cells. Finally, when multiple individual cells are formed on a common monolithic polymer support film it is difficult to check the quality of individual cells and remove deficient cell regions. Thus variations in cell quality over an expansive surface may jeopardize the entire module.
It has become well known in the art that the efficiencies of certain promising thin film photovoltaic junctions such as those based on copper-indium-gallium-diselenide or cadmium telluride can be substantially increased by high temperature treatments. These treatments involve temperatures at which even the most heat resistant and expensive plastics suffer rapid deterioration. Therefore, from a practical standpoint these thin film photovoltaic semiconductors are most often deposited on ceramic, glass, or metal substrates to support the thin film junctions. Use of a glass or ceramic substrates generally restricts one to batch processing and handling difficulty. Use of a metal foil, such as stainless steel, as a substrate allows continuous roll-to-roll manufacture of cell structure over an expansive surface. However, despite the fact that use of a metal foil allows high temperature processing in roll-to-roll fashion, the subsequent interconnection of individual cells effectively into an interconnected array has proven difficult, in part because the metal foil substrate is electrically conducting. For example, the monolithic integration techniques possible with insulating substrates are not possible using metal foil substrates, since the common substrate is a conducting metal and would not permit the required electrical isolation of individual cells prior to electrical series interconnection.
Many manufacturers of thin film photovoltaic devices supported on metal foil substrates choose to subdivide the material into discrete cells prior to assembly into an interconnected array. Typical of these methods is that which replicates the “string and tab” legacy approaches used for module assembly of crystalline silicon cells. Here the expansive metal foil/photovoltaic structure is subdivided into individual cells, typically of dimensions about 15 cm. by 15 cm, before subsequent assembly via the “string and tab” approach described above.
Some attempts have been advanced to achieve the advantages of continuous production of interconnected modules using continuously produced cell structure supported on a metal foil substrate. U.S. Pat. No. 4,746,618 to Nath et al. teaches a design and process to achieve interconnected arrays using roll-to-roll processing of a metal web substrate such as stainless steel. U.S. Pat. No. 4,746,618 is hereby incorporated in its entirety by reference. The process includes multiple operations of cutting, selective deposition, material removal and riveting. These operations add considerably to the final interconnected array cost. U.S. Pat. No. 5,385,848 to Grimmer teaches roll-to-roll methods to achieve integrated series connections of adjacent thin film photovoltaic cells supported on an electrically conductive metal substrate. U.S. Pat. No. 5,385,848 is hereby incorporated in its entirety by reference. The process includes mechanical or chemical etch removal of a portion of the photovoltaic semiconductor and transparent top electrode to expose an upper surface portion of the electrically conductive metal substrate. The exposed metal serves as a contact area for interconnecting adjacent cells. These material removal techniques are troublesome for a number of reasons. First, many of the chemical elements involved in the best photovoltaic semiconductors are expensive and environmentally unfriendly. This removal subsequent to controlled deposition involves containment, dust and dirt collection and disposal, and possible cell contamination. This is not only wasteful but considerably adds to expense since a significant amount of the valuable photovoltaic semiconductor is lost to the removal process. Ultimate module efficiencies are further compromised in that the spacing between adjacent cells grows, thereby reducing the effective active collector area for a given module area.
Yet another approach to achieve current collection and series interconnections among multiple cells while maintaining the flexible characteristic of many thin film structures is represented by the teachings of Yoshida et al. in U.S. Pat. No. 5,421,908. U.S. Pat. No. 5,421,908 is hereby incorporated in its entirety by reference. An embodiment of the current collection teachings of Yoshida et al. is presented in FIGS. 2A through 2C. Yoshida et al. teach a process wherein a conductive rear“1st” electrode 94 is first deposited using vacuum processing onto a polymeric film 96 as shown in FIG. 2A. Through holes 92 are then formed through the laminate. As shown in FIG. 2B, an overlaying amorphous silicon photovoltaic film 97 and TCO “2nd” electrode layer 98 are deposited on the laminate and through the holes. As shown in FIG. 2C, electrical communication between a top surface TCO “2nd” electrode 98 and a backside“3rd” electrode 99 is made through the holes when the “3rd” electrode 99 is deposited on the rear of the structure, as shown in FIG. 2C. The rear “3rd” electrode 99 is deposited by vacuum processing which also may coat the side walls of the holes. As Yoshida et al. teach, the “2nd” and “3rd” electrode layers in the holes are insulated from the “1st” electrode 94 by the high resistance of the amorphous silicon semiconductor layer. One readily realizes that an appropriate insulating layer would have to coat the holes to separate these electrodes should a semiconductor of lower resistivity be employed. To complete a series connection to an adjacent cell, the “3rd” electrode 99 of a first cell is further electrically joined to rear “1st” electrode 94 of an adjacent cell through additional holes between scribe lines separating the adjoining cells.
The through holes taught by Yoshida represent means to transport current from the topside surface of a photovoltaic cell to a conductive material (“3rd” electrode) located remote from the top surface. Thus the through holes of Yoshida et al. are functionally equivalent to the silver grid lines and wire forms discussed above in relation to FIGS. 1A through 1C.
A number of manufacturing and performance problems are intrinsic with the method and structure taught by Yoshida et al. First, both the “1st” rear cell electrode and the “3rd” backside electrode are relatively thin, being formed by vacuum sputtering. Vacuum processing is expensive and in practice yields relatively thin deposits. As taught by Yoshida et al. deposits of less than one half micrometer were employed. This relatively low practical thickness limits the current carrying ability of the deposited metal and thereby restricts the size of the individual cells. Moreover, absent additional conductive fill material in the holes, the connection between the backside “3rd” electrode and the rear “1st” electrode of adjacent cells is achieved only through a very restricted cross section. This is a result of the limited access to the “1st” electrode, since there is no access to the broad surface regions of the “1st” electrode, only its edge surface. The primary support for the Yoshida structure is the insulating polymeric film, which thus must be present during formation of the semiconductors. While perhaps acceptable when manufacturing amorphous silicon cells taught by Yoshida et al., it may be unlikely that the films taught would be suitable for the heat treatment requirements of other notable thin film semiconductors. The hole density taught by Yoshida et al. is quite large (15 mm centers) adding to complexity. However, even with the large hole density, the resistive losses expected in current transport to the holes would be quite large given the sheet resistance of a normal TCO. To address this issue, Yoshida et al. proposed a structure combining printed silver ink grid lines leading to a reduced number of through holes (see for example FIG. 28A of U.S. Pat. No. 5,421,908). Finally, many individual cells are formed on a common monolithic support film using the Yoshida et al. teaching. There is no way to check the quality of individual cells and remove deficient cell regions. Thus variations in cell quality over an expansive surface jeopardize the entire module.
One target feature associated with the integration of multiple cells is to create a multi-cell module which is flexible. Flexibility of a module promises a number of important benefits. For example, flexibility would allow convenient packaging of expansive area modules prior to deployment in an outer space application. This flexibility goal for modules was addressed using rigid crystalline silicon cells by Lebrun and Kurth in U.S. Pat. Nos. 3,553,030 and 4,019,924 respectively. U.S. Pat. Nos. 3,553,030 and 4,019,924 are incorporated herein in their entirety by this reference. These patents teach positioning of rigid crystal silicon cells on a flexible support substrate and interconnecting using conductive connectors applied to the substrate. Unfortunately because of the rigid nature of the crystal silicon cells, these modules taught by Lebrun and Kurth only achieved flexible interconnections between rigid silicon regions and therefore were not flexible over the entirety of the modular expanse. Thus the modules proposed by Lebrun and Kurth would not be suitable for many processes such as those associated with continuous web handling or roll-to-roll processes which require the structure to be substantially flexible over the entire expanse of its surface. Other applications such as many building integrated photovoltaic structures and unique installations cannot be considered when the structure does not have substantial flexibility over the entirety of its surface.
Thus there remains a need for manufacturing processes and articles which allow facile production of photovoltaic semiconductor structures while also offering unique means to achieve effective integrated connections to result in final modular array.
In a somewhat removed segment of technology, a number of electrically conductive fillers have been used to produce electrically conductive polymeric materials. This technology generally involves mixing of a conductive filler such as silver particles with the polymer resin prior to fabrication of the material into its final shape. Many choices exist for the conductive filler, including those comprising metals such as silver, copper and nickel, those comprising conductive metal oxides such as indium-tin oxide and zinc oxide, intrinsically conductive polymers, graphite, carbon black and the like. Conductive fillers may have high aspect ratio structure such as metal fibers such as stainless steel fibers or metallized polymer fibers. Other high aspect ratio materials such as metal flakes or powder, or highly structured carbon blacks may be appropriate, with the choice based on a number of cost/performance considerations. More recently, fine particles of intrinsically conductive polymers have been employed as conductive fillers within a resin binder. Electrically conductive polymers have been used as bulk thermoplastic compositions, or formulated into paints and inks. Their development has been spurred in large part by electromagnetic radiation shielding and static discharge requirements for plastic components used in the electronics industry. Other known applications include resistive heating fibers and battery components and production of conductive patterns and traces. The characterization “electrically conductive polymer” covers a very wide range of intrinsic resistivities depending on the filler, the filler loading and the methods of manufacture of the filler/polymer blend. Resistivities for filled electrically conductive polymers may be as low as 0.00001 ohm-cm. for very heavily filled silver inks, yet may be as high as 10,000 ohm-cm or even more for lightly filled carbon black materials or other “anti-static” materials. “Electrically conductive polymer” has become a broad industry term to characterize all such materials. In addition, it has been reported that recently developed intrinsically conducting polymers (absent conductive filler) may exhibit resistivities comparable to conductive metals.
In yet another separate technological segment, coating plastic substrates with metal electrodeposits has been employed to achieve decorative effects on items such as knobs, cosmetic closures, faucets, and automotive trim. The normal conventional process actually combines two primary deposition technologies. The first is to deposit an adherent metal coating using chemical (electroless) deposition to first coat the nonconductive plastic and thereby render its surface highly conductive. This electroless step is then followed by conventional electroplating. ABS (acrylonitrile-butadiene-styrene) plastic dominates as the substrate of choice for most applications because of a blend of mechanical and process properties and ability to be uniformly etched. The overall plating process comprises many steps. First, the plastic substrate is chemically etched to microscopically roughen the surface. This is followed by depositing an initial metal layer by chemical reduction (typically referred to as “electroless plating”). This initial metal layer is normally copper or nickel of thickness typically one-half micrometer. The object is then electroplated with metals such as bright nickel and chromium to achieve the desired thickness and decorative effects. The process is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully prepared parts and designs. In addition, the many steps employing harsh chemicals make the process intrinsically costly and environmentally difficult. Finally, the sensitivity of ABS plastic to liquid hydrocarbons has prevented certain applications. ABS and other such polymers have been referred to as “electroplateable” polymers or resins. This is a misnomer in the strict sense, since ABS (and other nonconductive polymers) are incapable of accepting an electrodeposit directly and must be first metallized by other means before being finally coated with an electrodeposit. The conventional technology for electroplating on plastic (etching, chemical reduction, electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47, or Arcilesi et al., Products Finishing, March 1984.
Many attempts have been made to simplify the process of electroplating on plastic substrates. Some involve special techniques to produce an electrically conductive film on the surface. Typical examples of this approach are taught by U.S. Pat. No. 3,523,875 to Minklei, U.S. Pat. No. 3,682,786 to Brown et. al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive film produced was then electroplated. None of these attempts at simplification have achieved any recognizable commercial application.
A number of proposals have been made to make the plastic itself conductive enough to allow it to be electroplated directly thereby avoiding the “electroless plating” process. It is known that one way to produce electrically conductive polymers is to incorporate conductive or semiconductive fillers into a polymeric binder. Investigators have attempted to produce electrically conductive polymers capable of accepting an electrodeposited metal coating by loading polymers with relatively small conductive particulate fillers such as graphite, carbon black, silver or nickel powder or flake or small metal coated forms such as metal coated mica. When considering polymers rendered electrically conductive by loading with electrically conductive fillers, it may be important to distinguish between “microscopic resistivity” and “bulk” or macroscopic resistivity”. “Microscopic resistivity” refers to a characteristic of a polymer/filler mix considered at a relatively small linear dimension of for example 1 micrometer or less. “Bulk” or “macroscopic resistivity” refers to a characteristic determined over larger linear dimensions. To illustrate the difference between “microscopic” and “bulk, macroscopic” resistivities, one can consider a polymer loaded with conductive fibers at a fiber loading of 10 weight percent. Such a material might show a low “bulk, macroscopic” resistivity when the measurement is made over a relatively large distance. However, because of fiber separation (holes) such a composite might not exhibit consistent “microscopic” resistivity. When producing an electrically conductive polymer intended to be electroplated, one should consider “microscopic resistivity” in order to achieve uniform, “hole-free” deposit coverage. Thus, it may be advantageous to consider conductive fillers comprising those that are relatively small, but with loadings sufficient to supply the required conductive contacting. Such fillers include metals such as silver in the form of powders or flake, metal coated particles such as mica or spheres, particles comprising conductive metal oxides such as indium-tin oxide and zinc oxide, fine particles of intrinsically conductive polymers, graphite powder and conductive carbon black and the like. Heavy loadings of such filler may be sufficient to reduce volume resistivity to a level where electroplating may be considered.
However, attempts to make an acceptable electroplateable polymer using the small conductive fillers alone encounter a number of barriers. First, the most conductive fine metal containing fillers such as silver are relatively expensive. The loadings required to achieve the particle-to-particle proximity to achieve acceptable conductivity increases the cost of the polymer/filler blend dramatically. The metal containing fillers are accompanied by further problems. They tend to cause deterioration of the mechanical properties and processing characteristics of many resins. This significantly limits options in resin selection. All polymer processing is best achieved by formulating resins with processing characteristics specifically tailored to the specific process (injection molding, extrusion, blow molding, printing etc.). A required heavy loading of metal filler severely restricts ability to manipulate processing properties in this way. A further problem is that metal fillers can be abrasive to processing machinery and may require specialized screws, barrels, and the like.
Another major obstacle involved in the electroplating of electrically conductive polymers is a consideration of adhesion between the electrodeposited metal and polymeric substrate (metal/polymer adhesion). In most cases sufficient adhesion is required to prevent metal/polymer separation during extended environmental and use cycles. Despite being electrically conductive, a simple metal-filled polymer offers no assured bonding mechanism to produce adhesion of an electrodeposit since the metal filler particles may be encapsulated by the resin binder or oxide, often resulting in a resin-rich or oxide “skin”.
A number of methods to enhance electrodeposit adhesion to electrically conductive polymers have been proposed. For example, etching of the surface prior to plating can be considered. Etching can be achieved by immersion in vigorous solutions such as chromic/sulfuric acid. Alternatively, or in addition, an etchable species can be incorporated into the conductive polymeric compound. The etchable species at exposed surfaces is removed by immersion in an etchant prior to electroplating. Oxidizing surface treatments can also be considered to improve metal/plastic adhesion. These include processes such as flame or plasma treatments or immersion in oxidizing acids.
In the case of conductive polymers containing finely divided metal, one can propose achieving direct metal-to-metal adhesion between electrodeposit and filler. However, here the metal particle surface may be shielded by an aforementioned resin or oxide “skin”. To overcome this effect, one could propose methods to remove the “skin”, exposing active metal filler to bond to subsequently electrodeposited metal. For the reasons described above, electrically conductive polymers employing metal fillers have not been widely used as bulk substrates for electroplateable articles. Nevertheless, revived efforts and advances have been made recently to accomplish electroplating onto printed conductive patterns formed by silver filled inks and pastes. In addition, such metal containing polymers have found considerable applications as inks or pastes in production of printed conductive traces for electrical circuitry, antennas etc.
Another approach to impart adhesion between conductive resin substrates and electrodeposits is incorporation of an “adhesion promoter” at the surface of the electrically conductive resin substrate. This approach was taught by Chien et al. in U.S. Pat. No. 4,278,510 where maleic anhydride modified propylene polymers were taught as an adhesion promoter. Luch, in U.S. Pat. No. 3,865,699 taught that certain sulfur bearing chemicals could function to improve adhesion of initially electrodeposited Group VIII metals.
An additional obstacle confronting practical electroplating onto electrically conductive polymers is the initial “bridge” of electrodeposit onto the surface of the electrically conductive polymer. In electrodeposition, the substrate to be plated is often made cathodic through a pressure contact to a highly conductive member under cathodic potential. However, if the contact resistance is excessive or the substrate is insufficiently conductive, the electrodeposit current favors the highly conductive member to the point where the electrodeposit will not bridge to the substrate.
Moreover, a further problem is encountered even if specialized racking or cathodic contact successfully achieves electrodeposit bridging to the substrate. Many of the electrically conductive polymers have resistivities far higher than those of typical metal substrates. Also, many applications contemplate electroplating onto a thin printed conductive ink pattern of traces or “fingers”. The dry conductive ink thickness is typically less than 25 micrometer and often less than 6 micrometer. The conductive polymeric pattern may be relatively limited in the amount of electrodeposition current which it alone can convey. Thus, the conductive polymeric substrate pattern does not cover almost instantly with electrodeposit as is typical with metallic substrates. Except for the most heavily loaded and highly conductive polymer substrates, a large portion of the electrodeposition current must pass back through the previously electrodeposited metal growing laterally over the surface of the conductive plastic substrate. In a fashion similar to the bridging problem discussed above, the electrodeposition current favors the electrodeposited metal and the lateral growth can be extremely slow and erratic. This restricts the size and“growth length” of the conductive ink pattern, increases plating costs, and can also result in large non-uniformities in electrodeposit integrity and thickness over the pattern.
This lateral growth is dependent on the ability of the substrate to convey current. This, the thickness and resistivity of a conductive polymeric ink pattern can be defining factors in the ability to achieve satisfactory electrodeposit coverage rates. When dealing with selectively electroplated patterns long thin metal traces are often desired, deposited on a relatively thin electrically conductive polymer substrate patterns. These factors of course often work against achieving the desired result.
This coverage rate problem likely can be characterized by a continuum, being dependent on many factors such as the nature of the initially electrodeposited metal, electroplating bath chemistry, the nature of the polymeric binder and the resistivity of the electrically conductive polymeric substrate. As a “rule of thumb”, the instant inventor estimates that coverage rate issue would demand attention if the resistivity of a bulk conductive polymeric substrate rose above about 0.001 ohm-cm. Alternatively, as a “rule of thumb” appropriate for conductive thin film substrate patterns, coverage rate issues may require attention if the substrate pattern to be plated has a surface “sheet” resistance of greater than about 0.05 ohm per square.
The least expensive (and least conductive) of the readily available conductive fillers for plastics are carbon blacks. Attempts have been made to electroplate electrically conductive polymers using carbon black loadings. Examples of this approach are the teachings of U.S. Pat. Nos. 4,038,042, 3,865,699, and 4,278,510 to Adelman, Luch, and Chien et al. respectively.
Adelman taught incorporation of conductive carbon black into a polymeric matrix to achieve electrical conductivity required for electroplating. The substrate was pre-etched in chromic/sulfuric acid to achieve adhesion of the subsequently electroplated metal. A fundamental problem remaining unresolved by the Adelman teaching is the relatively high resistivity of carbon loaded polymers. The lowest “microscopic resistivity” generally achievable with carbon black loaded polymers is about 1 ohm-cm. This is about five to six orders of magnitude higher than typical electrodeposited metals such as copper or nickel. Thus, the electrodeposit bridging and coverage rate problems described above remained unresolved by the Adelman teachings.
Luch in U.S. Pat. No. 3,865,699 and Chien et al. in U.S. Pat. No. 4,278,510 also chose carbon black as a filler to provide an electrically conductive surface for the polymeric compounds to be electroplated. The Luch U.S. Pat. No. 3,865,699 and the Chien U.S. Pat. No. 4,278,510 are hereby incorporated in their entirety by this reference. However, these inventors further taught inclusion of materials to increase the rate of electrodeposit coverage or the rate of metal deposition on the polymer. These materials can be described herein as “electrodeposit growth rate accelerators” or “electrodeposit coverage rate accelerators”. An electrodeposit coverage rate accelerator is a material functioning to increase the electrodeposition coverage rate over the surface of an electrically conductive polymer independent of any incidental affect it may have on the conductivity of an electrically conductive polymer. In the embodiments, examples and teachings of U.S. Pat. Nos. 3,865,699 and 4,278,510, it was shown that certain sulfur bearing materials, including elemental sulfur, can function as electrodeposit coverage or growth rate accelerators to overcome problems in achieving electrodeposit coverage of electrically conductive polymeric surfaces having relatively high resistivity or thin electrically conductive polymeric substrates having limited current carrying capacity.
In addition to elemental sulfur, sulfur in the form of sulfur donors such as sulfur chloride, 2-mercapto-benzothiazole, N-cyclohexyle-2-benzothiaozole sulfonomide, dibutyl xanthogen disulfide, and tetramethyl thiuram disulfide or combinations of these and sulfur were identified. Those skilled in the art will recognize that these sulfur donors are the materials which have been used or have been proposed for use as vulcanizing agents or accelerators. Since the polymer-based compositions taught by Luch and Chien et al. could be electroplated directly they could be accurately defined as directly electroplateable resins (DER). These directly electroplateable resins (DER) can be generally described as electrically conductive polymers with the inclusion of a growth rate accelerator.
Specifically for the present invention, specification, and claims, directly electroplateable resins, (DER), are characterized by the following features:                (a) presence of an electrically conductive polymer;        (b) presence of an electrodeposit coverage rate accelerator;        (c) presence of the electrically conductive polymer and the electrodeposit coverage rate accelerator in the directly electroplateable composition in cooperative amounts required to achieve direct coverage of the composition with an electrodeposited metal or metal-based alloy.        
In his patents, Luch identified elastomers such as natural rubber, polychloroprene, butyl rubber, chlorinated butyl rubber, polybutadiene rubber, acrylonitrile-butadiene rubber, styrene-butadiene rubber etc. as suitable for the matrix polymer of a directly electroplateable resin. Other polymers identified by Luch as useful included polyvinyls, polyolefins, polystyrenes, polyamides, polyesters and polyurethanes.
When used alone, the minimum workable level of carbon black required to achieve “microscopic” electrical resistivities of less than 1000 ohm-cm. for a polymer/carbon black mix appears to be about 8 weight percent based on the combined weight of polymer plus carbon black. The “microscopic” material resistivity generally is not reduced below about 1 ohm-cm. by using conductive carbon black alone. This is several orders of magnitude larger than typical metal resistivities.
It is understood that in addition to carbon blacks, other well known, highly conductive fillers can be considered in DER compositions. Examples include but are not limited to metallic fillers such as silver powder or flake, metal coated forms such as metal coated mica or glass spheres, graphite powder and conductive metal oxides. In these cases the more highly conductive fillers can be used to augment or even replace the conductive carbon black. Furthermore, one may consider using intrinsically conductive polymers to supply the required conductivity. In this case, it may not be necessary to add conductive fillers to the polymer.
The “bulk, macroscopic” resistivity of fine conductive particle filled polymers can be further reduced by augmenting the filler with additional highly conductive, high aspect ratio forms such as metal containing fibers. This can be an important consideration in the success of certain applications. Furthermore, one should realize that incorporation of non-conductive fillers may increase the “bulk, macroscopic” resistivity of conductive polymers loaded with finely divided conductive fillers without significantly altering the “microscopic resistivity” of the conducive polymer “matrix” encapsulating the non-conductive filler particles.
Regarding electrodeposit coverage rate accelerators, both Luch and Chien et al. in the above discussed U.S. patents demonstrated that sulfur and other sulfur bearing materials such as sulfur donors and vulcanization accelerators function as electrodeposit coverage rate accelerators when using an initial Group VIII metal electrodeposit “strike” layer. Thus, an electrodeposit coverage rate accelerator need not be electrically conductive, but may be a material that is normally characterized as a nonconductor. The coverage rate accelerator need not appreciably affect the conductivity of the polymeric substrate. As an aid in understanding the function of an electrodeposit coverage rate accelerator the following is offered:                a. A specific conductive polymeric structure is identified as having insufficient current carrying capacity to be directly electroplated in a practical manner.        b. A material is added to the conductive polymeric material forming said structure. Said material addition may have insignificant affect on the current carrying capacity of the structure (i.e. it does not appreciably reduce resistivity or increase thickness).        c. Nevertheless, inclusion of said material greatly increases the speed at which an electrodeposited metal laterally covers the electrically conductive surface.It is contemplated that a coverage rate accelerator may be present as an additive, as a species absorbed on a filler surface, or even as a functional group attached to a polymer chain. One or more growth rate accelerators may be present in a directly electroplateable resin (DER) to achieve combined often synergistic results.        
A hypothetical example is an extended trace of conductive ink having a dry thickness of two micrometer. Such inks typically comprise a conductive filler such as silver, nickel, copper, conductive carbon etc. The limited thickness of the ink may reduce the current carrying capacity of this trace thus preventing direct electroplating in a practical manner. However, inclusion of an appropriate quantity of a coverage rate accelerator may allow the conductive trace to be directly electroplated in a practical manner.
One might expect that other Group 6A elements, such as oxygen, selenium and tellurium, could function in a way similar to sulfur. In addition, other combinations of electrodeposited metals such as copper and appropriate coverage rate accelerators may be identified. It is important to recognize that such an electrodeposit coverage rate accelerator is important in order to achieve direct electrodeposition in a practical way onto polymeric substrates having low conductivity or very thin electrically conductive polymeric substrates having restricted current carrying ability.
It has also been found that the inclusion of an electrodeposit coverage rate accelerator promotes electrodeposit bridging from a discrete cathodic metal contact to a DER surface. This greatly reduces the bridging problems described above.
Due to multiple performance problems associated with their intended end use, none of the attempts identified above to directly electroplate electrically conductive polymers or plastics has ever achieved any recognizable commercial success. Nevertheless, the current inventor has persisted in personal efforts to overcome certain performance deficiencies associated with the initial DER technology. Along with these efforts has come a recognition of unique and eminently suitable applications employing the DER technology. Some examples of these unique applications for electroplated articles include solar cell electrical current collection grids, electrodes, electrical circuits, electrical traces, circuit boards, antennas, capacitors, induction heaters, connectors, switches, resistors, inductors, batteries, fuel cells, coils, signal lines, power lines, radiation reflectors, coolers, diodes, transistors, piezoelectric elements, photovoltaic cells, emi shields, biosensors and sensors. One readily recognizes that the demand for such functional applications for electroplated articles is relatively recent and has been particularly explosive during the past decade.
It is important to recognize a number of important characteristics of directly electroplateable resins (DERs) which facilitate the current invention. One such characteristic of the DER technology is its ability to employ polymer resins and formulations generally chosen in recognition of the fabrication process envisioned and the intended end use requirements. A very wide choice of polymer resins and blends, additives and fillers is available with the directly electroplateable resin (DER) technology. Functional combinations of polymers and additives, such as curatives, stabilizers, and adhesion promoters can be widely chosen. In order to provide clarity, examples of some such fabrication processes are presented immediately below in subparagraphs 1 through 9.                (1) Should it be desired to electroplate an ink, paint, coating, or paste which may be printed or formed on a substrate, a good film forming polymer, for example a soluble resin such as an elastomer, can be chosen to fabricate a DER ink (paint, coating, paste etc.). For example, in some embodiments thermoplastic elastomers having an olefin base, a urethane base, a block copolymer base or a random copolymer base may be appropriate. In some embodiments the coating may comprise a water based latex. Other embodiments may employ more rigid film forming polymers. The DER ink composition can be tailored for a specific process such flexographic printing, rotary silk screening, gravure printing, flow coating, spraying etc. Furthermore, additives can be employed to improve the adhesion of the DER ink to various substrates. One example would be tackifiers.        (2) Very thin DER traces often associated with electrical traces such as current collector grid structures can be printed and then electroplated due to the inclusion of the electrodeposit growth rate accelerator.        (3) Should it be desired to cure the DER substrate to a 3 dimensional matrix, an unsaturated elastomer or other “curable” resin may be chosen.        (4) DER inks can be formulated to form electrical traces on a variety of flexible substrates. For example, should it be desired to form electrical structure on a laminating film, a DER ink adherent to the sealing surface of the laminating film can be effectively electroplated with metal and subsequently laminated to a separate surface.        (5) Should it be desired to electroplate a fabric, a DER ink can be used to coat all or a portion of the fabric intended to be electroplated. Furthermore, since DER's can be fabricated out of the thermoplastic materials commonly used to create fabrics, the fabric itself could completely or partially comprise a DER. This would eliminate the need to coat the fabric.        (6) Should one desire to electroplate a thermoformed article or structure, DER's would represent an eminently suitable material choice. DER's can be easily formulated using olefinic materials which are often a preferred material for the thermoforming process. Furthermore, DER's can be easily and inexpensively extruded into the sheetlike structure necessary for the thermoforming process.        (7) Should one desire to electroplate an extruded article or structure, for example a sheet or film, DER's can be formulated to possess the necessary melt strength advantageous for the extrusion process.        (8) Should one desire to injection mold an article or structure having thin walls, broad surface areas etc. a DER composition comprising a high flow polymer can be chosen.        (9) Should one desire to vary adhesion between an electrodeposited DER structure supported by a substrate the DER material can be formulated to supply the required adhesive characteristics to the substrate. For example, the polymer chosen to fabricate a DER ink can be chosen to cooperate with an “ink adhesion promoting” surface treatment such as a material primer or corona treatment.        
All polymer fabrication processes require specific resin processing characteristics for success. The ability to “custom formulate” DER's to comply with these changing processing and end use requirements while still allowing facile, quality electroplating is a significant factor in the teachings of the current invention.
Another important recognition regarding the suitability of DER's for the teachings of the current invention is the simplicity of the electroplating process. Unlike many conventional electroplated plastics, DER's do not require a significant number of process steps prior to actual electroplating. This allows for simplified manufacturing and improved process control. It also reduces the risk of cross contamination such as solution dragout from one process bath being transported to another process bath. The simplified manufacturing process will also result in reduced manufacturing costs.
Another important recognition regarding the suitability of DER's for the teachings of the current invention is the wide variety of metals and alloys capable of being electrodeposited. Deposits may be chosen for specific attributes. Examples may include copper or silver for conductivity and nickel or chromium for corrosion resistance, and tin or tin based alloys for solderability.
Yet another recognition of the benefit of DER's for the teachings of the current invention is the ability they offer to selectively electroplate an article or structure. The articles of the current invention often consist of metal patterns selectively positioned in conjunction with insulating materials. Such selective positioning of metals is often expensive and difficult. However, the attributes of the DER technology make the technology eminently suitable for the production of such selectively positioned metal structures. As will be shown in later embodiments, it is often desired to electroplate a polymer or polymer-based structure in a selective manner. DER's are eminently suitable for such selective electroplating.
Yet another recognition of the benefit of DER's for the teachings of the current invention is the ability they offer to continuously electroplate an article or structure. As will be shown in later embodiments, it is often desired to continuously electroplate articles. DER's are eminently suitable for such continuous electroplating. Furthermore, DER's allow for selective electroplating in a continuous manner.
Yet another recognition of the benefit of DER's for the teachings of the current invention is their ability to withstand the pre-treatments often required to prepare other materials for plating. For example, were a DER to be combined with a metal, the DER material would be resistant to many of the pre treatments such as cleaning which may be necessary to electroplate the metal.
Yet another recognition of the benefit of DER's for the teachings of the current invention is that the desired plated structure often requires the plating of long and/or broad surface areas. As discussed previously, the coverage rate accelerators included in DER formulations allow for such extended surfaces to be covered in a relatively rapid manner thus allowing one to consider the use of electroplating of conductive polymers.
These and other attributes of DER's may contribute to successful articles and processing of the instant invention. However, it is emphasized that the DER technology is but one of a number of alternative metal deposition or positioning processes suitable to produce many of the embodiments of the instant invention. Other approaches, such as printing of conductive resin formulations, metal spraying, etching metal foils, stamping metal foils, laminating metal foils, positioning and affixing metal patterns, electroless metal deposition, vacuum metal evaporation and sputtering, or electroplating onto various conductive ink patterns such as those comprising silver may be suitable alternatives. These choices will become clear in light of the teachings to follow in the remaining specification, accompanying figures and claims.
In order to eliminate ambiguity in terminology, for the present invention the following definitions are supplied:
While not precisely definable, for the purposes of this specification, electrically insulating materials may generally be characterized as having electrical resistivities greater than 10,000 ohm-cm. Also, electrically conductive materials may generally be characterized as having electrical resistivities less than 10,000 ohm-cm. A subset of conductive materials, electrically resistive or semi-conductive materials may generally be characterized as having electrical resistivities in the range of 0.001 ohm-cm to 10,000 ohm-cm. The term “electrically conductive polymer or resin” as used in the art and in this specification and claims extends to materials of a very wide range of resistivities from about 0.00001 ohm-cm. to about 10,000 ohm-cm and higher.
An “electroplateable material” is a material having suitable attributes that allow it to be coated with a layer of electrodeposited material, either directly or following a preplating process.
A “metallizable material” is a material suitable to be coated with a metal deposited by any one or more of the available metallizing process, including chemical deposition, vacuum metallizing, sputtering, metal spraying, sintering and electrodeposition.
“Metal-based” refers to a material or structure having at least one metallic property and comprising one or more components at least one of which is a metal or metal-containing alloy.
“Alloy” refers to a substance composed of two or more intimately mixed materials.
“Group VIII metal-based” refers to a substance containing by weight 50% to 100% metal from Group VIII of the Periodic Table of Elements.
A “metal-based foil” refers to a thin structure of metal or metal-based material that may maintain its integrity absent a supporting structure. Generally, metal of thickness greater than about 2 micrometers may have this characteristic (i.e. 2 micrometers, 10 micrometers, 25 micrometers, 100 micrometers, 250 micrometers).
A “film” refers to a thin material form that is not necessarily self supporting.
In this specification and claims, the terms “monolithic” or “monolithic structure” are used as is common in industry to describe an object that is seamless and made or formed into or from a single item.
A “continuous” process is one wherein a continuous form of a material component is supplied to the process. The material feed can be continuous motion or repetitively intermittent, and the output is timed to remove product either by continuous motion or repetitively intermittent according to the rate of input.
A “roll-to-roll” process is one wherein a material component is fed to the process from a roll of material and the output of the process is accumulated in a roll form.
The “machine direction” is that direction in which material is transported through a process step.
The term “multiple” is used herein to mean “two or more”.
A “web” is a thin, flexible sheetlike material form often characterized as continuous in a length direction.
“Sheetlike” characterized a structure having surface dimensions far greater than the thickness dimension.
“Substantially planar” characterizes a surface structure which may comprise minor variations in surface topography but from an overall and functional perspective can be considered flat.
The terms “upper” and “top” surfaces of structure refer to those surfaces of structure facing toward the light incident side of the structure and are thus depicted in the drawing embodiments as facing upward.
The terms “lower” or “bottom” surface refer to surfaces facing away from the light incident side of the structure.